Analysis And Application Study Of Positive Temperature Coefficient Wavy Fin Heaters For Electric Vehicles In Winter

A positive temperature coefficient(PTC)heater is an automotive heating device widely used in electric vehicles to ensure battery conversion efficiency in winter,but the heater consumes a significant amount of battery capacity.Therefore,it is necessary to improve the performance of the PTC heater to meet the maximum mileage of electric vehicles in winter.In this thesis,the heat transfer performance and resistance performance of the wavy fins are investigated by CFD method and the structural parameters are optimally selected for the electric vehicle PTC wavy fin heater.Finally,the influence of ambient temperature on the thermal management strategy of electric vehicles in winter is discussed and the adaptability of different thermal management strategies is analyzed.The main research findings of the thesis are as follows:In this thesis,we propose to replace straight fins with wavy fins and develop a PTC heater with heating performance above 7 k W level.In the flow range of 100-500 kg/h,this study investigates the effects of four parameters,including fin pitch(F_p),fin amplitude(F_a),fin thickness(F_t)and ripple number(R_n),on the heat transfer performance and resistance performance of the heater through numerical simulation.Colburn j-factor and friction coefficient f-factor were selected as the performance evaluation criteria for heat transfer and flow.It is found that decreasing F_p increases the overall heat transfer area of the heater and reduces the pressure loss due to the obstruction of the hot rod boundary,so the heating performance and resistance performance are improved.Increasing F_a strengthens the local heat transfer performance,but thickens the entire fin temperature boundary layer,which means that increasing Fa is detrimental to the overall heat transfer performance,and in addition,the high pressure area at the trough is subsequently increased and the resistance performance deteriorates.Increasing F_t reduces the fin thermal resistance,but the change in F_t has little effect on the heat transfer performance,but increases the pressure loss at the front of the fins.With the increase of R_n,the fin heat transfer area and air mixing are improved and the heat transfer performance is significantly improved,which also increases the pressure drop and local pressure drop along the fins and the resistance performance deteriorates sharply.Based on the multi-response Taguchi design method,the influence factors are F_p,F_a,F_tand R_n,with four levels of each factor.Heating capacity,AGF and pressure drop is used as three response,and the comprehensive weighted score is taken as the overall performance criterion.The results of the range analysis and analysis of variance(ANOVA)using Minitab software show that reducing F_p and choosing a smaller R_n can improve the overall performance,the best overall performance when F_t=0.3mm,F_a is a secondary factor and it reflects the volatility.The optimal combination of F_p=2mm,F_a=0.8mm,F_t=0.3mm and R_n=2was obtained by discussing the design parameters.The new wavy fin heater has a heating performance of 7.19 k W and a pressure drop of 18.3 Pa.At the standard flow rate,the wavy fin heater has 16.91%higher heating capacity,7.95%higher pressure drop and 33.40%higher AGF than the plate fin heater.Based on the thermodynamic cycle and energy multi-stage utilization theory,PTC heating,pure heat pump heating and heat pump PTC heating strategies are established to study the effect of ambient temperature on the thermal management strategy,taking the passenger compartment and battery pack of electric vehicles as the research objects.The results show that the PTC strategy has a block heating rate,but the system temperature stabilization time is longer and the energy consumption is high.the PTC is not suitable as an electric vehicle thermal management strategy in cold winter.The heat pump PTC strategy is suitable for heating under full working conditions in winter,and the system is the best performer in terms of heat production,energy consumption ratio and temperature control time.Compared with the PTC strategy,the average temperature control time of the battery pack decreases by 300s and the average temperature control time of the cabin decreases by 600s.In summary,the numerical simulation and analysis method based on Taguchi’s design proposed in this thesis can realize the optimal design of PTC heaters,in addition,the numerical simulation based on system simulation can also realize the adaptation analysis of thermal management strategies for electric vehicles in winter.The method provides a reference for the prediction of thermal management performance and the design of related parameters for early products.